[WSGR LETTERHEAD]

                                                                     EXHIBIT 5.1



                                [Opinion of WSGR]
                                  July 22, 2003


Komag, Incorporated
1710 Automation Parkway
San Jose, California 95131

      RE: REGISTRATION STATEMENT ON FORM S-8

Ladies and Gentlemen:

      We have examined the Registration Statement on Form S-8 to be filed by you
with the Securities and Exchange Commission on or about July 22, 2003 (the
"Registration Statement") in connection with the registration under the
Securities Act of 1933, as amended, of 750,000 shares of your Common Stock (the
"Shares") to be issued under your Amended and Restated 2002 Qualified Stock Plan
and your Amended and Restated 2002 Employee Stock Purchase Plan (collectively,
the "Plans"). As your counsel in connection with this transaction, we have
examined the proceedings taken and are familiar with the proceedings proposed to
be taken by you in connection with the issuance of the Shares.

      It is our opinion that, when the shares are issued pursuant to options and
rights granted in the manner described in the Plans, and pursuant to the
agreements which accompany each option grant, the Shares will be legally and
validly issued, fully-paid and non-assessable.

      We consent to the use of this opinion as an exhibit to the Registration
Statement, and further consent to the use of our name whenever appearing in the
Registration Statement and any amendments thereto.

                                        Very truly yours,

                                        WILSON SONSINI GOODRICH & ROSATI
                                        Professional Corporation

                                        /s/ Wilson Sonsini Goodrich & Rosati